For deep drawing applications in the manufacture of appliances and the like, prior art workers have used both rimming and aluminum killed steels. Rimming steel (core killed steel) is cheaper to manufacture and has cleaner surface properties both in the ingot form and as rolled. A small amount of temper rolling after annealing will eliminate as-annealed yield point elongation (YPE), but the steel will still age at ordinary room temperature (about 23.degree. C.) in about two months, resulting in the return of objectionable yield point elongation.
The present invention will be described in terms of a pickling or surface treatment process applied to the vats and doors of dishwashing machines. This is to be considered exemplary only and is not intended to constitute a limitation on the present invention. It will be evident that teachings of the present invention will be equally applicable to the vats and doors of clothes washing machines and other porcelain coated appliances and the like.
In an exemplary prior art surface preparation treatment for the subsequent porcelain coating of dishwasher vats and doors fabricated from rimming steel, the vats and doors were suspended from a conveyor and sprayed with a series of liquids. Specifically, an exemplary prior art surface preparation treatment involved the following sequence of steps:
(1) The suspended vats and doors were sprayed with an aqueous alkaline cleaning solution (such as a sodium hydroxide solution), at a temperature of about 180.degree. F. (about 82.degree. C.). The purpose of this step was to remove oil and other residue having accumulated during fabrication and transportation to the porcelain plant.
(2) The suspended vats and doors were thereafter subjected to a first water rinsing step to remove the alkaline cleaning solution. This rinsing step included the spraying of the vats and doors first with recirculating warm water, followed by spraying with recirculating cold water, and finally followed by spraying with fresh water.
(3) The doors and vats were thereafter sprayed with an aqueous ferric sulfate solution having a concentration of from about 3% to about 3.5% and a temperature of from about 165.degree. F. to about 170.degree. F. (about 74.degree. C. to about 77.degree. C.). The ferric sulfate solution was made by injecting a ferrous sulfate solution with a hydrogen peroxide solution having a concentration of from about 5% to about 8%. This step is an etching phase of the process.
(4) The vats and doors were thereafter sprayed with a recirculating water rinse, as a second water rinse to flush away the ferric sulfate solution.
(5) The vats and doors were next sprayed with an aqueous sulfuric acid solution having a concentration of from about 5% to about 6% and a temperature of about 160.degree. F. (about 71.degree. C.). The purpose of this step was again to etch the surface of the steel.
(6) The suspended vats and doors were thereafter subject to a third water rinse comprising recirculating and fresh water rinse sprays to eliminate any acid remaining from step 5.
(7) An aqueous nickel sulfate solution, having a nickel concentration from about 2 to about 2.5 oz./gal., and a temperature of about 170.degree. F. (about 77.degree. C.) was sprayed on the suspended vats and doors to deposit a coating of nickel on the steel. The nickel coating is important for good adherence of the porcelain on the steel.
(8) The suspended vats and doors were thereafter treated with a rinsing spray of an aqueous sulfuric acid solution having a very low concentration of about 0.35% to about 0.45%. The purpose of this step is to remove iron (rust) that may have dissipated on the surface of the rimming steel vats and doors during the application of the nickel sulfate solution (step 7).
(9) The suspended doors and vats were then sprayed with an alkaline neutralizer such as a soda ash solution, to terminate the corrosive action of any of the previous solutions.
(10) The strip preparation treatment was completed by a final water rinse comprising a spray of recirculated water followed by a spray of deionized water.
After the above surface preparation steps, the suspended parts were subjected to a heated drying step, followed by a conventional porcelain coating operation. The coating operation may be of any appropriate and well known type, such as dipping the parts in wet porcelain, or subjecting the parts to a direct, dry powder application. Thereafter, the parts were subjected to a conventional baking step at high temperature.
Although the exemplary prior art surface preparation process generally worked well for preparing rimming steel parts for porcelain coating, some problem with "out-gassing" (a pitting condition in the porcelain which may lead to premature rusting of the steel therebeneath) was encountered.
Recently, with the impropvements in its manufacturing process and properties, a great deal of interest has been shown in the use of aluminum killed steel. Aluminum killed steel will be permanently non-aging after a small amount of temper rolling following an anneal, so long as it is not exposed to elevated temperatures prior to forming or deep drawing. Some difficulties have been encountered in preparing the surfaces of parts made from aluminum killed steel for porcelain coating. For example, the above outlined strip preparation process has been found to adversely affect the aluminum killed steel, making it unsuitable for subsequent porcelain coating. Problems manifest themselves such as, for example, pitting of the metal surface. Gas or atmosphere is trapped in the metal surface pitts by the porcelain coating which, when baked, is itself subject to out-gassing and increased consequent pitting.
The present invention is based upon the discovery that, with proper modification, a surface preparation process can be provided which will enable satsifactory porcelain coating of parts made from aluminum killed steel. Additional advantages are also achieved. For example, in the practice of the above described prior art process, steps 3 through 5 result in a loss of the base metal of the parts being surface treated. In the practice of the present invention this base metal loss is markedly reduced. It has further been discovered that, in the practice of the present invention, the amount of nickel deposited to provide an acceptable adhesion layer is less than that required by prior art practice, representing a significant cost savings, since the amount of nickel sulfate required is greatly reduced. Finally, accelerated rust tests show that the process of the present invention has largely eliminated the out-gassing and related rust problems inherent in the prior art process outlined above.